26.4 Groups of Stars
When Giovanni Riccioli used a telescope like this one to
observe a star in the Big Dipper, he discovered two stars
that orbit each other.
•A group of stars that appear
to form a pattern as seen
from Earth is called a
constellation.
–The stars in a constellation
are generally not close to one
another.
–They just happen to lie in
the same general direction of
the sky as seen from Earth.
Red Sea
26.4 Groups of Stars
• A star system is a group of two or more stars that are
held together by gravity.
• Astronomers have determined that more than half of all
stars are members of star systems.
• A star system with two stars is called a binary star.
– The two stars orbit each other.
– Sometimes the smaller star in a binary star is too dim to be
seen easily from Earth but can still be detected from the
motion of the other star.
– If one star passes in front of the other, blocking some of the
light from reaching Earth, the star system is called an
eclipsing binary.
26.4 Groups of Stars
Star Clusters
•There are three basic kinds of star clusters: open
clusters, associations, and globular clusters.
•Studying star clusters is useful because all the stars
formed together in the same nebula, so they are
about the same age and the same distance from
Earth.
–Astronomers plot the stars of a cluster on an H-R diagram
to estimate the cluster’s age.
26.4 Groups of Stars
Star Clusters
A. The Pleiades are an open star cluster that
is visible to the unaided eye.
B. 47 Tucanae is a spectacular globular
cluster that is visible in southern skies.
26.4 Groups of Stars
Star Clusters
•Open clusters have a disorganized/loose
appearance and contains no more than a few
thousand stars that are well spread out.
–Often contain bright supergiants, gas and dust clouds.
•Associations are temporary groupings of bright,
young stars.
–Gravity from nearby stars breaks these apart
–Typically larger than open clusters
26.4 Groups of Stars
Star Clusters
•A globular cluster is a large group of older stars.
–Usually lack enough gas and dust to form new stars
–Spherical
–Dense concentration of stars in the center
–Can contain more than 1 million stars
–Usually do not have short-lived blue stars (these stars
have already died out)
•Astronomers estimate that the oldest globular
clusters are about 12 billion years old.
–Thus, the universe must be at least that old.
26.4 Groups of Stars
Galaxies
•Astronomers classify galaxies into four main
types: spiral, barred-spiral, elliptical, and
irregular.
•A galaxy is a huge group of individual stars, star
systems, star clusters, dust, and gas bound together
by gravity.
• There are billions of galaxies in the universe
• The largest galaxies consist of more than 1 trillion stars
• Galaxies vary widely in size and shape.
26.4 Groups of Stars
Spiral and Barred-Spiral
Galaxies
Spiral galaxies have a bulge of
stars at the center, with arms
extending outward like a pinwheel.
• Spiral arms contain gas, dust, and
many bright young stars
• The Milky Way is a spiral galaxy
• Some have a bar through the
center with the arms extending
outward, called barred-spiral
galaxies.
spiral galaxy in the
constellation Coma
Berenices
26.4 Groups of Stars
Elliptical Galaxies
Elliptical galaxies are spherical or oval, with no
spiral arms
• Wide range of sizes
• Very little gas or dust between stars
• Contain only old stars
B. barred-spiral galaxy in the Fornax cluster
C.
Elliptical galaxy M87
26.4 Groups of Stars
Irregular Galaxies
•Irregular galaxies have a disorganized appearance
–Many young stars and large amounts of gas and dust.
–A small fraction of all galaxies are known as irregular
–Come in many shapes
–Typically smaller than other types of galaxies
–Are often located near larger galaxies
D. Irregular galaxy with many areas of star formation
26.4 Groups of Stars
Galaxies
The Milky Way Galaxy
•Has an estimated 200 - 400 billion stars and a
diameter of more than 100,000 light years.
•Every star you can see with the unaided eye is in our
galaxy.
•The solar system lies in the Milky Way’s disk within a
spiral arm, about two thirds of the way from the
center.
26.4 Groups of Stars
Galaxies
In a side view, the Milky Way appears as a flat disk
with a central bulge. An overhead view of the Milky
Way shows its spiral shape.
Location of
solar system
Central bulge
Nucleus
Overhead View of Our Galaxy
Disk of spiral arms
containing mainly
young stars
Central bulge
containing mainly
older stars
Halo containing
Nucleus
oldest stars
About 100,000 light-years
Side View of Our Galaxy
26.4 Groups of Stars
Galaxies
•The Milky Way’s flattened disk shape is caused by
its rotation.
•The sun takes about 220 million years to complete
one orbit around the galaxy’s center.
•Recent evidence suggests that there is a massive
black hole at our galaxy’s center.
•Stars are forming in the galaxy's spiral arms.
26.4 Groups of Stars
Galaxies
Quasars
•By studying their spectra, astronomers have
determined that quasars are the enormously bright
centers of distant, young galaxies.
•What makes a quasar so bright?
–Quasars produce more light than 100s of galaxies the size
of the Milky Way.
–The most likely explanation involves matter spiraling into a
super-massive black hole with the mass of a billion suns.
26.4 Groups of Stars
Assessment Questions
1. A constellation is
a.
b.
c.
d.
two stars that orbit each other.
a star system with more than two stars.
an open cluster of stars that are close to one another.
a group of stars that appear to form a pattern.
ANS:
D
26.4 Groups of Stars
Assessment Questions
2. A large group of older stars without sufficient gas
and dust to form new stars is a(n)
a.
b.
c.
d.
open cluster.
galaxy.
association.
globular cluster.
ANS:
D
26.4 Groups of Stars
Assessment Questions
3. What type of galaxy is the Milky Way?
a.
b.
c.
d.
spiral galaxy
barred-spiral galaxy
elliptical galaxy
irregular galaxy
ANS:
A